1. Field of Invention
The present invention relates to a gas jet actuator device operated by air pressure utilizing the Coanda effect to generate propulsing forces to impart controlled vibrations to an object to which the device is secured, and further to the method of vibrating an object using this device.
The gas jet actuator device of the present invention was developed particularly, but not exclusively, for testing the mechanical properties of a human or robotic arm. The gas jet actuator device constitutes an instrument when placed in a system for measuring the effects of an object when vibrations are imparted by the actuator device.
2. Description of Prior Art
Knowledge of the joint mechanical properties of the human arm during posture and movement is important for testing and inferring control strategies. Until recently, this knowledge has been limited to single-joint movements because instrumentation has not been available to apply appropriate spatial perturbations to the arm and to measure accurately the resulting displacements. The device of the present invention was developed to perturb arbitrary and natural multi-joint human arm movements, to measure accurately the force of perturbation and to allow the application of nonlinear stochastic system identification techniques to characterize the joint mechanical properties.
Previous arm studies have primarily used electric motors for perturbations. Advantages of electric motors are that power amplifiers and servo systems are readily available commercially, and that accurate position transducers are easily incorporated to measure rotor motion. Disadvantages are that they are bulky and heavy relative to their force or torque, and that they can only apply low-frequency perturbations because of the large rotor inertia and limitations in the power amplifiers. One consequence is that electric motors have been primarily applied to single-joint perturbation. In one known configuration, the a upper arm and forearm are strapped or cast into a single degree-of-freedom mechanical linkage, and the motor axis directly exerts torque on the coincident linkage and elbow joints. In another known configuration, the elbow point is fixed and a force is exerted directly on the wrist by an attached rod or cable running to the motor. Planar two-dimensional perturbations have been applied to the hand or wrist with two-link, parallel drive mechanisms driven by electric motors.
Perturbations have also been applied by hydraulic actuators, which offer advantages of much higher force or torque over electric motors for a given size. Depending on the servo-valve design, they may also provide a higher frequency output. They have been applied to the elbow and to the ankle.
Neither hydraulic actuators nor electric motors lend themselves readily towards a three-dimensional perturbation device. Although transmission elements such as rods or cables are conceivable, the dynamics of such elements are likely to confound the arm dynamics and to limit the perturbation bandwidth.
Recently, pneumatic thrusters have been devised as perturbation devices where compressed air is the power source and hydraulic spool valves control the air flow. Since gas jet nozzles can be mounted on a cuff attached to the wrist, they offer the possibility of multi-dimensional perturbations without significant constraints on arm movement. The tubing running to the cuff is light and flexible and does not impede movement, and the expelled air is of course not an environmental problem. These gas jet systems therefore represent a major advance in instrumentation because the experimentally imposed arm movement constraint is eliminated. A disadvantage of the spool valve design in these gas jets is spool mass and the resulting limitation on the system frequency bandwidth to about 20 Hz.
For any perturbation device designed to infer the human elbow's joint mechanical properties, a high frequency bandwidth is essential. There is reason to believe that these properties are nonlinear and time-varying and a high-frequency stochastic input is the best input to identify them. Even if a linear model is assumed, a high frequency is required to identify inertia reliably.
The gas jet actuator device of the present invention uses, as the primary control, a fluidic switching device, based on the Coanda effect. Such design greatly reduces the mass of mechanical moving parts and enhances the frequency bandwidth dramatically. This gas jet is intrinsically a bistable device that can generate arbitrary binary force sequences, such as pseudorandom binary sequences (PRBS), colored white noise, and Walsh functions. PRBS have been used as they are the most efficient implementation of a Gaussian white noise signal. An additional part of the system is the Optotrak (Registered Trademark--Northern Digital Inc., Waterloo, Ontario), a three-dimensional motion tracking system whose resolution is 0.05mm. The recent emergence of such high-accuracy measurement systems is essential for unrestrained arm movement studies in general and for our perturbation studies in particular.